US marines and mechanised infantry drove to within 80 kilometres (50 miles) of Saddam's seat of power on Wednesday, forming an arch of men and armour across the city's southern approaches, cutting key communication lines and inflicting severe damage on some of the Iraqi military's elite divisions.

But as they got closer to the city, officers ordered soldiers to move up to the next level of precaution against chemical or biological weapon attacks by donning rubber boots along with the protective clothing they have worn almost every day since the US-British invasion began on March 21.

It was the equivalent of asking soldiers to fight under a hot sun in heavy raincoats and thick Wellingtons. US commanders remain convinced that Saddam possesses weapons of mass destruction and may be planning to use them once coalition forces advance beyond a certain point on the map.

Each soldier also has a gas mask slung around his waist, and has been trained to put it on within nine seconds of the alarm being raised.

"There may be a trigger line where the regime sees a sufficient threat to use weapons of mass destruction," Brigadier General Vincent Brooks told reporters at the Central Command forward headquarters here.

"That's the red line. It's a conceptual line across which there may be a decision made. That's why we attack the regime's methods of communications, that's why we attack those who would make decisions.

"It's about preventing the action as much as possible."

Brooks last week warned that intelligence officers had received "indications through a variety of sources" that the Iraqi regime had issued "first orders" to use chemical or biological weapons if the coalition forces crossed the line.

"We have seen chemical protective equipment in a number of areas south of where we thought that red line might be," he said last week, adding however that he had not seen "anything saying that an order has been given to fire."

US and British forces have discovered large supplies of chemical protective suits and gas masks in captured Iraqi military stores.

Allied officials have said their biggest fear is a possible chemical or biological attack despite warnings to Iraqi commanders to refuse any such orders or face war crimes charges.

Iraqi Information Minister Mohammed Said al-Sahhaf last week dismissed US suggestions that the discovery of protective clothing in captured Iraqi barracks meant that Iraq possessed such arms.

"In war, the combatant expects the enemy to use anything at all," he said, adding that Iraq did not rule out the coalition resorting to "stupidities" due to "demoralisation caused by their defeat."

Chief United Nations arms inspector Hans Blix has said it is too early to say whether Iraq has chemical or biological weapons.

Asked about the discovery by US troops of protective suits and antidotes to chemical weapons in Iraq, Blix replied: "Everyone knows that they were importing atropine ... (but) one would have to wait for more solid information" before drawing any conclusions.

Blix said he believed that even if the Iraqis had proscribed weapons of mass destruction, they would refrain from using them.

"In the first place, the world would say they were liars, and in the second place it would change the attitude of the world about the armed conflict," he said.

"That's a guess, and I may be wrong," he added.

The UN Monitoring, Verification and Inspection Commission withdrew from Iraq on March 18, two days before US President George W. Bush declared the start of hostilities.

SPACE.WIRE